1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of driving the same. A liquid crystal display device conducts a light and dark display by utilizing a voltage applied to a liquid crystal layer interposed between substrates to change a polarized state, a scattered state, or wavelength characteristics of light passing through the liquid crystal layer.
In the present specification, a thin film transistor (TFT) refers to a semiconductor device having a semiconductor layer, a gate electrode, a source electrode, and a drain electrode.
2. Description of the Related Art
Liquid crystal display devices have been used widely for portable equipment, personal computers, and the like in view of their light weight and low power consumption.
In a liquid crystal display device, a field sequential system has been paid attention to, in which light sources of three primary colors (red, green, and blue) are successively lighted to conduct a color display. The field sequential system does not need a color filter, so a high-precision display can be expected.
Regarding the field sequential system, it is proposed that light sources are successively and continuously lighted with varying light emission colors (Monthly FPD Intelligence Press Journal, 1999. 2, pp. 66-69). According to this system, when light emission colors of the light sources are changed, it is required to set the entire screen at a black level so as to prevent mixed color of the light sources in the respective pixels.
It is also proposed that a light source is lighted after response of liquid crystal is completed in a screen (Shunsuke Kobayashi, Color Liquid Crystal Display, Sangyo Tosho, Japan, p. 127). According to this system, light sources are intermittently lighted, so that a complete black display can be achieved when the light sources are not lighted. This allows an impulse system that is a driving system of a cathode ray tube (CRT) to be achieved even in a liquid crystal display device, and this system can be expected to prevent a residual image peculiar to a liquid crystal display device.
The problems to be solved by the invention will be described below.
Note that, in the present specification, a TFT formed in a pixel portion is referred to as a “pixel TFT”.
Furthermore, in the present specification, signal lines having addresses S1 to Sm, scanning lines having addresses G1 to Gn, and pixels disposed in the vicinity of crossing points between the signal lines and the scanning lines are formed in pixel portions. Each pixel has a pixel TFT. A gate electrode of the pixel TFT is connected to a scanning line, and a source electrode thereof is connected to a signal line. The address of each pixel is represented by an address of a signal line connected to a source electrode of a pixel TFT, and an address of a scanning line connected to a gate electrode thereof. For example, when a pixel TFT is connected to a signal line in an i-th column and a scanning line in a j-th row, the address of a pixel having this pixel TFT is (i, j).
Furthermore, a pixel electrode is formed so as to be connected to a drain electrode of a pixel TFT, and an opposing electrode is opposed to the pixel electrode. Liquid crystal is interposed between the pixel electrode and the opposing electrode via an alignment film. Liquid crystal is switched in accordance with a potential difference between the pixel electrode and the counter electrode.
In dot-sequential driving, a period from a time when a first scanning line is selected to a time when an n-th scanning line is selected is referred to as a “scanning period of scanning lines”. Applying a predetermined potential (e.g., +8 volts to +11 volts) to a scanning line for the purpose of activating a semiconductor layer is referred to as “selecting a scanning line”. A period for selecting a scanning line is referred to as a “scanning line selection period”.
More specifically, the “scanning period of scanning lines” refers to a period required from the beginning of selection of a first scanning line to the end of selection of an n-th scanning line. “Selecting a scanning line” refers to applying a gate pulse to a pixel TFT connected to a scanning line, thereby bringing a conducting state between a source and a drain of the pixel TFT connected to the scanning line. Furthermore, a selection period of a scanning line refers to a period for selecting one scanning line, and the “scanning period of scanning lines” is obtained by multiplying the selection period of a scanning line by n times.
Furthermore, selecting a signal line refers to applying a signal voltage to a signal line, and applying a potential of the signal line to a pixel TFT connected to the signal line.
Furthermore, a period from a time when a potential required for an image display is applied to a pixel electrode of a pixel TFT having an address (1,1) to a time when one monochromic image is formed is referred to as a “sub-frame period”. A period from a time when a potential required for an image display is applied to a pixel electrode of a pixel TFT having an address (1,1) to a time when one color image is formed is referred to as a “frame period”.
According to the field sequential system, a frame period in which a color image is displayed includes a sub-frame period for forming a red image, a sub-frame period for forming a blue image, and a sub-frame period for forming a green image.
FIG. 7 shows a timing chart of the field sequential system in which light sources are lighted intermittently. According to the field sequential system, a cycle (T) of one frame period is 16.6 msec., and a cycle (T/3) of a sub-frame period is 5.5 msec.
In dot-sequential driving, one scanning line is selected, and signal lines are successively selected by a shift register of a source driver, to thereby apply a potential of a signal line to a pixel electrode of each pixel TFT connected to a selected scanning line. The sub-frame period is divided into a standby period 301, a scanning line selection period 302, a liquid crystal response period 303, and a lighting period 304 of light sources. The standby period refers to a period from a time when one frame period starts to a time when a scanning line connected a pixel TFT is selected. The liquid crystal response period refers to a period in which liquid crystal responses in accordance with a potential of a pixel electrode. A scanning period 308 of scanning lines is obtained by multiplying a scanning line selection period by the number (n) of scanning lines.
In the scanning line selection period 302, a scanning line is selected, and a pixel electrode of a pixel TFT connected to the scanning line is successively supplied with a potential of a signal line in accordance with a desired gray-scale. In the liquid crystal response period 303, optical response of liquid crystal is completed. In the lighting period 304 of light sources, light sources are lighted intermittently, whereby a first light emission color 305, a second light emission color 306, and a third light emission color 307 are successively entered into a liquid crystal display device. For example, as the first light emission color, a red color is used. As the second light emission color, a green light is used. As the third light emission color, a blue light is used. However, when the light sources are lighted intermittently, the liquid crystal response period 303 of the pixel TFTs connected to a first scanning line is different from that of the pixel TFTs connected to an n-th scanning line. When it takes a long period of time for liquid crystal to respond, or when the scanning period 308 of scanning lines is long, if it is attempted to light a light source after liquid crystal response is completed, the lighting period 304 of light sources becomes short, which decreases lightness.
According to the field sequential system, one important factor is a response time of liquid crystal. As the response time of liquid crystal becomes shorter, a lighting period of a light source can be made longer to conduct a light display.
Another important factor of the field sequential system is a scanning period of scanning lines. Assuming that there are n scanning lines, when a scanning period becomes longer, it takes a shorter period of time for a light source to be lighted after the application of a potential of a signal line to a pixel electrode toward the n-th scanning line. Therefore, before response of liquid crystal is completed, a light source is lighted. A gray-scale level is determined by the integral of lightness shown by liquid crystal when a light source is lighted. If a light source is lighted before response of liquid crystal is completed, a gray-scale level when a screen is displayed is changed. On the contrary, if a light source is lighted after response of liquid crystal is completed, a lighting period of a light source becomes shorter, which results in a dark display.
In liquid crystal display devices with a number of scanning lines, e.g., XGA (1024 pixels (horizontal direction)×768 pixels (vertical direction)), SXGA (1280 pixels (horizontal direction)×1024 pixels (vertical direction), the ratio of a scanning period of scanning lines in a sub-frame period is not negligible. In dot-sequential driving of the SXGA liquid crystal display device, even if a write time of a signal to one pixel is set to be 0.75 to 1.5 nsec, a scanning period of scanning lines is estimated to be 1 to 2 msec. Therefore, when a scanning period of scanning lines is removed from a sub-frame period (5.5 msec), only 3.5 to 4.5 msec remains. If liquid crystal is allowed to respond until desired lightness is obtained and a light source is lighted within this time, a lighting time of a light source becomes considerably short, making it difficult to conduct a light display.
In the present specification, optical response of liquid crystal is allowed to be completed as early as possible in driving of a liquid crystal display device of the field sequential system. Furthermore, a scanning period of scanning lines is shortened to decrease a ratio of a standby period 301 in a sub-frame period.
More specifically, in the present specification, in the field sequential system, a sum of a standby period 301 and a liquid crystal response period 303 is shortened, and a lighting period 304 of a light source is prolonged, whereby a light display is conducted.